Spiral galaxies are characterised by their distinctive spiral arms of stars, gas, and dust that extend outward from a central bulge. They evolve through complex processes that span, or at least were thought to span billions of years, beginning as vast clouds of hydrogen gas that collapse under gravity. As these galaxies mature, they develop their spiral patterns through differential rotation, where inner regions rotate faster than outer ones, creating density waves that trigger star formation along the arms. Over time the galaxies evolve further with the mergers of smaller galaxies that ultimately shape the galaxies we see today.
The Whirlpool Galaxy, a classic spiral galaxy with a merging event underway (Credit : NASA and ESA)
Our own Milky Way is one such galaxy that has slowly transformed as it moves through its evolutionary journey. However a recent study by a team of astronomers at the University of Geneva (UNIGE) has discovered the most distant spiral galaxy to date that challenges us to rethink galactic evolution. The discovery of the ultra-massive spiral galaxy was published in Astronomy and Astrophysics following observations from the James Webb Space Telescope.
Artist impression of the James Webb Space Telescope
The galaxy has been named “Zhúlóng” after the mythological dragon that creates day and night by opening and closing its eyes, symbolising the connection between light and cosmic time. It’s the most distant spiral galaxy ever found, existing just 1 billion years after the Big Bang. Despite its early age, this galaxy is surprisingly well-developed.
‘‘We named this galaxy Zhúlóng, meaning ‘Torch Dragon’ in Chinese mythology. In the myth, Zhúlóng is a powerful red solar dragon that creates day and night by opening and closing its eyes” – Dr. Mengyuan Xiao, postdoctoral researcher at the Department of Astronomy of the Faculty of Science of UNIGE and lead author of the study.
Zhúlóng is remarkable for its similarity to the Milky Way in shape, size, and stellar content. Its disk stretches more than 60,000 light-years across—comparable to our galaxy—and contains over 100 billion solar masses worth of stars, also comparable to the Milky Way. This makes it one of the most convincing Milky Way-like galaxies ever discovered but from such an early era it forces us to rethink how large, organised spiral galaxies develop, especially so soon after the Universe began.
Image of the night sky above Paranal, Chile taken by ESO astronomer Yuri Beletsky showing the light from the combined stars in the Milky Way. The laser beaming out from Yepun, Unit Telescope number 4 points directly at the Galactic Centre (Credit : ESO/Y. Beletsky)
The team discovered Zhúlóng using JWST’s PANORAMIC survey, which uses a special “pure parallel” mode allowing the telescope to capture wide areas of space efficiently. This approach is especially useful for finding massive galaxies that are extremely rare. They now plan to conduct follow-up observations using JWST and the Atacama Large Millimetre Array to confirm Zhúlóng’s properties and learn more about how it formed.
Zhúlóng’s discovery represents a shift in our understanding of galaxy evolution, challenging long-held assumptions about the timescales required for spiral structure formation. The existence of such a mature, massive spiral galaxy just one billion years after the Big Bang suggests that galactic evolution processes may occur much faster than previously thought. As more wide-area surveys continue with JWST, astronomers expect to find additional early spiral galaxies, providing new insights into how galaxies evolved in the early Universe.
Source : The most distant twin of the Milky Way ever observed